Lighting

Flood Lighting – Armstrong, Sykes and Dennis Lighting

Why light bulbs are so strong?

The glass of an electric light bulb is not much thicker than the paper of this page, yet it withstands a strong grip when you push it into a light fitting. The explanation for this lies mainly in the bulb’s shape, which exploits the eggshell principle.

Aeons ago, Nature found a solution to the problem of preventing eggs from being crushed by the weight of the hen bird as she sat on the nest to incubate them. The solution was the characteristic egg shape, which provides structural strength, to withstand all-round pressure even with a thin shell. (If the shell were too thick, the chick inside would not be able to peck its way out.) Light bulbs (and eggs) have a rounded profile over the whole surface. When you grip a bulb, the force you apply is transmitted in all directions away from the point of contact by the curve of the glass.

This results in the force being distributed over a wide area, and no excessive stress being set up at any one point.

Making light bulbs is an intricate and highly automated factory process in which the bulbs are blown into shape in moulds from a continuous ribbon of molten glass.
A vital component of the bulbs is the filament, a coil of tungsten wire one-hundredth of a millimetre thick. This is the part that becomes white-hot and produces the light when electricity flows through. It is mounted on a glass stem and clamped to the end of thicker wires that pass through the stem to the base of the bulb.

When the stem is inserted in the bulb, any oxygen in the bulb is eliminated (otherwise it would cause the coil to oxidise, greatly reducing its life). The bulb is then filled with an argon/nitrogen mixture. It is sealed and the metal cap is cemented in place. A modern bulb-making machine can produce 30 bulbs in a few minutes, each able to pour out light for at least 1000 hours. Gradually, however, the metal filament evaporates. Eventually it will break and the light will fail.